Icom ID-51A Service Manual - Page 14

4-2 TRANSMIT CIRCUITS TX AF CIRCUIT (LOGIC UNIT) The AF signal from the internal or external microphone (MIC signal) is applied to the audio processor LSI (IC25), through the ATT (R190 and R198). The audio processor LSI contains 16-bit liner CODEC, MIC AMP, audio filters, D/A converter, and so on. • When operating in the DV mode The applied signal is amplified by the MIC AMP, and passed through the external MIC mute SW (IC16, pins 7, 1), and then further amplified by the ALC AMP. The amplified signal is applied to the liner CODEC, through the LPF (R186, C174 and C177), to be encoded into a digital audio signal. The digital audio signal is processed by the DSP (IC18), and then passed through the LPF (R125 and C124), mode SW (IC390, Q390) and LPF (IC25) ,and then applied to the D/A converter (IC25) to be adjusted in level. The level-adjusted signal is applied to the modulation circuits. • When operating in the FM/FM-N mode The amplified signal is passed through the MIC mute SW (IC16, pins 7, 1), HPF (R130, C160 and C161), BPF (IC25), LPF (IC25) and mode SW (IC390, pins 8, 9), and then applied to the D/A converter (IC25) to be adjusted in level, through the mode SW (IC390, pins 11, 10) and LPF (IC25). The level-adjusted signal is applied to the modulation circuits. MODULATION CIRCUITS (MAIN UNIT) The MIC signal from the TX AF circuits is applied to the VHF VCO (Q311, D304-D306) or UHF VCO (Q27, D10-D12) as a modulation signal. The modulation signal is applied to D10 to obtain GMSK (For DV mode) or Frequency Modulation (For FM mode). The modulated VCO output signal is amplified by the buffer (Q200) and LO AMP (Q201), and then applied to the TX AMP circuits as the TX signal, through the LO SW (D14). • MODULATION CIRCUIT TX AF circuit -> Q311, D304,D305,D306 Q200 VHF VCO BUFF Q201 LO AMP TX AMP circuit -> Q27, D10,D11,D12 UHF VCO VCO A UNIT MAIN UNIT • TX AF CIRCUIT MAIN UNIT EXTMIC IC25 J5 MC1 ATT MIC AMP IC16 MMUTE FM SW HPF DV ALC AMP BPF LPF IC390 LPF LINEAR CODEC DMO MODE LPF DMOD SW LPF IC18 DSP LOGIC UNIT D/A VCOMOD 4 - 5